Shielding arrangement for transformer



Oct. 4, 1966 E. M. BARR 3,277,416

SHIELDING ARRANGEMENT FOR TRANSFORMER Filed Dec. 4, 1962 5 Sheets-Sheet1 l ,|4 {I 3: i- 1 0b i 2 34 r-""| I I INVENTOR. ELLIOTT M. BARR UnitedStates Patent 3,277,416 SHIELDING ARRANGEMENT FOR TRANSFORMER Elliott M.Barr, Brighton, N.Y., assignor to Taylor Instrument Companies,Rochester, N.Y., a corporation of New York Filed Dec. 4, 1962, Ser. No.242,304 10 Claims. (Cl. 336-84) This invention relates to transformersand the general object of the invention is to provide a transformer ofimproved interference-rejection properties.

Another general object of the invention is to provide a transformerhaving shielded windings, that is readily produced in quantity, with aminimum of hand-crafting and individual attention, yet with such qualityand precision of construction as to fulfiill rather exacting performancespecifications, particularly as regards interference rejection.

Other, more specific objects of the invention will emerge from thedetailed description and claims appended hereto.

FIGURE 1 illustrates a measuring system, or the like, in combinationwith my novel transformer;

FIGURES 2 and 3 respectively illustrate an exploded shielded windingunit or module according to the invention, and the same unit inassembled condition;

FIGURE 4 illustrates a transformer, according to the invention, made upof winding units of the type shown in FIGURE 3;

FIGURE 5 illustrates a winding in the transformer of FIGURE 4;

FIGURE 6 is a fragmentary diagram, along the lines of FIGURE 1,illustrating electrical interconnection of the winding modules used inthe transformer of FIG- URE 4;

FIGURES 7 and 8 respectively illustrate a cover that may be used toprovide supplementary shielding for the winding unit of FIGURE 3, and afragmentary view of such winding unit with said cover in place thereon;and

FIGURES 9, 10 and 11 elaborate a concept relied upon in expressing astructural feature of the invention.

FIGURE 1 illustrates, quite generally, my novel transformer incombination with such entities as may be provided in order that thewhole defines what amounts to a measuring system.

Thus, a transducer T having a DC. output (a thermocouple, say, sensingthe temperature in a process of some sort), is provided, the of which isapplied to a reference unit R which in turn provides a DC. bias for theE.M.F of the transducer T, and produces a net DC. output combining theDC. bias and transducer output.

At this point, it is convenient to chop the DC. signal and modulate anA.C. carrier with the DC. signal, or otherwise operate on the DC. signalto the end of providing an A.C. signal, some characteristic of which isrepresentative of the amplitude of the DC. signal at any given instantmore or less. For this purpose I provide the switch S, which isessentially a single pole, double throw switch having fixed contacts 1and 2, and movable contact 3. Suitable means (not shown) are providedfor vibrating contact 3 between contacts 1 and 2, to make and break thelatter cyclically. Typically, the movable contact 3 is driven at theA.C. mains frequency (e.g., 60 cycles per second).

To convert the action of switch S to a modulation a transformer L, ofconstruction peculiar to my invention, is provided. Transformer L iscomprised of the continuous closed core 10 and a set of windings,including wind ings 11, 21, 31, of which windings 11 and 21 form acompound primary winding and winding 31 is a secondary winding. Core 10,in practice, would be built up from a number of laminations having theform of core 10, as illusunit spacer for use 3,277,416 Patented Oct. 4,1966 trated in FIGURE 1, which laminations would normally be made ofsome ferrous or like material having the usual magnetic propertiesassociated with iron, steel, etc.

Winding 11 consists of two distinct induction coils 12 and 13, the turnsof which are so oriented that a DC. current therein in a given senseinduces a magnetic flux in core 10 that is proportional to the totalampere turns represented by the coils 12 and 13.

Winding 21 consists of two distinct induction coils 22 and 23, the turnsof which are so oriented that a DC. current therein in a given senseinduces a magnetic flux in core 10 that is proportional to the totalampere turns represented by the coils 12 and 13.

Windings 11 and 21 are made identical in total ampere turns, etc., sothat the same given current in either winding will produce the samegiven magnetic flux in core 10. In order to apply this characteristic tothe use of modulation, coils 23 and 13 are provided with a commonterminal 4 and this common terminal 4 is connected to one side of theoutput of reference device R, those ends of windings 11 and 21 beingchosen for connection to ter minal 4 such that with respect to currentflow into or out of terminal 4, the ampere turns of the one winding willhave a sense opposite to the sense of the ampere turns of the otherwinding.

Accordingly, modulation is made possible by connecting the remaining endof winding 11 to fixed contact 1 of switch S, the remaining end ofwinding 21 to fixed contact 2 of switch S, and the remaining side of theoutput of reference device R to movable contact 3 of switch S.

Supposing a DC. voltage to exist between terminal 4 and contact 3, itwill be observed that also supposing windings 11 and 21 to have the sameampere turns and supposing the contacts 1, 2 and 3 of switch S to beshorted together, the winding sense depicted in FIGURE 1 indicates thatthe net core flux created, as a result of current flow simultaneouslythrough windings 11 and 21, would be Zero, since the induction action ofthe said windings are in mutual opposition; (note arrows on legs 10::and 10b, indicating this). Therefore, considering switch S as FIGURE 1actually depicts it, moving contact 3 first to one of contacts 1 and 2,and then to the other thereof, would result in first energizing but oneof windings 11 and 21, and then in energizing solely the other thereof,with the further result that a finite value of core flux having a givensense would first arise only to be followed by a core flux in a senseopposite said given sense.

As FIGURE 1 indicates, secondary winding 31 consists of coils 32 and 33,the ampere turns of which are so directed that the inductivecharacteristic of the one coil aids that of the other. The repetition ofthe make-break cycle of switch S, as described above results in voltagesbeing induced in coils 32 and 33 by transformer action, and these coilvoltages would add to produce a voltage across the coils in series thatwould be the sum of the absolute magnitude of the individual coilvoltages at any given instant. The output voltage across winding 31 isan A.C. voltage of substantially square wave form.

I therefore provide suitable A.C. amplifying means, depicted asamplifier A, in FIGURE 1, for amplifying the output of winding 31, inorder to provide a suitable input for a load M connected to the outputof amplifier A. In view of what has already been said herein, by way ofillustration, as to transformer T and reference device R, load M may beconsidered essentially a motor having for its purpose the operation of avalue or other control element, or of a recorder, indicator, or otherexhibiting device. In the former case, a conventional arrangement wouldbe that valve or control element operation by motor M would change atemperature sensed by transducer T in such fashion that the DC. outputof the transducer would neutralize the DC bias of reference device R,thereby reducing the net output of device R to zero. In the latter case,the motor would operate not on the temperature sensed by transducer T,but on the reference device R, adjusting the bias of the latter toneutralize the of the transducer, thereby reducing the output of deviceR to zero, as in the said former case, and, at the same time, adjustingthe position of a pointer, recorder-stylus, or the like, to correspondto said temperature.

Functionally speaking, the foregoing is well known in the prior art, isa matter of routine practice, and, as to the modulation and transformeraspects thereof is considered to be desirable practice for a number ofreasons, including the fact that use of transformer coupling creates thepossibility of rejecting certain types of electrical interference at thetransformer. For example, at some point in the output circuitryconnected to secondary winding 31 there will be a circuit common, aground to one side of the AC. mains, or the like, which could couplethrough the signal circuit to a point of a potential differing from thatof the line voltage. Suppose, for example there is an output circuitground to one side of the AC. mains, and the transducer T is athermocouple and happens to contact the other side of the mains.Obviously, the transformer L is a barrier to completing a circuit acrossthe AC. mains via the signal circuit.

In practice, however, the matter is not so simple for the transformer isobviously not an infinite D.C. resistance, and necessarily has somecapacity coupling its primary and secondary windings together. It is,moreover, subject to the effect of stray fields, both electrostatic andelectromagnetic. These effects are normally dealt with by properinsulation, electrostatic shielding, magnetic shielding, and carefulattention to impedance relations, mode of arranging circuit connections,and so forth.

While some of these effects may be eliminated, or almost whollysuppressed, after a point, relatively little can be done with thecapacitance of the transformer, other than to arnange the elements ofthis capacitance in such a way as to reduce the net effect thereof inthe matter of coupling interference into the secondary circuit of thesystem, or to complicate the circuitry with active devices, such asamplifiers, arranged to neutralize transformer capacitance, and havingthat sole purpose, essentially.

One expedient for controlling the capacity of a transformer and forrejecting electrostatic influences is shielding. Such shielding is shownin FIGURE 1, wherein the broken lines denoted by numerals 14, 24 and 34represent electrostatic shields. For the present, each shield may beconsidered an envelope, made of DC. conductive material, such as metalfoil, metallized paint, metal mesh, or the like, defining an integral,equi-potential surface completely surrounding one of the windings 11, 21and 31, except where it is necessary to take the winding leads out ofthe shield to some point exterior to the transformer as a whole,indicated at 15, 25 and 35, which numerals denote lengths of tubularmetal braid, or the like, D.C.-connected to the corresponding shield. Itshould be added that the shielding is conveniently extended around thevarious primary leads, the modulator, and, in general, around anyimpedance in the primary circuit of the transformer, the whole shieldingsystem (except shield 34 and its extension 35 on the secondary side ofthe transformer) thus constituted being connected to some potential inthe primary circuit. The remainder of the shielding, namely shield 34and its extension 35, are connected to ground or equivalent, on thesecondary side. Conveniently, the primary shielding is left floating onone of the transducer terminals. Therefore, should a signal betweenground or equivalent, and the primary shielding arise (say thattransducer T is a thermocouple and it or one of its leads contacts apoint at a potential different from that of the shielding on thesecondary side of the transformer), a current will flow between shield34, on the one hand, and shields 14 and 24 on the other via thecapacitance between the former and each of the latter. With thearrangement of coils and shields shown in the drawing, this capacitativecurrent will be unable to induce any net flux in the core. For example,consider current flow between shields 14 and 34, in the sense of thearrows placed just over those portions of core legs 10a and lflb betweenshields 14 land 34. Here it is evident that any fluxcreated in leg 10aby the said current will be opposed by the flux created in leg 10b bythe current. Supposing the intershield current to be uniformlydistributed in the space between the shields, .and legs 10a and 10b tobe parallel and alike, the net flux thus created will be Zero.

The same follows if we consider the intershield current between shields24 and 34, having reference to the [arrows placed just over the portionsof legs 10a and 10b showing between shields 24 and 34, which arrowsindicate the sense of the current flow between shields 24 and 34,'whenthe current between shields 14 and 34 is as indicated supra. It will benoted that if either lead to contacts 1 and 2 of the modulator werebroken (which would change the modulating action half-Way instead offull-wave, and which is feasible) the operative remainder of thetransformer coupling would still remain unaffected by the interferingcurrent.

Likewise, it will be apparent that the leads of either one of windings11 and 21 could be interchanged (at contact 1 and terminal 4), and anA.C. signal to be measured placed directly across terminals 1 and 2. (Wemay suppose that we have modulated some external A.C. carrier inaccordance with the output of reference device R in preference tochopping said output in the manner shown, and still desire to couple themodulated A.C. carrier to the amplifier A via transformer L.)

As indicated, each individual coil of windings 13, 23 and 33circumscribes one or the other of legs 10a and 10b of core 10. Each ofshields 14, 24 and 34, therefore, necessarily circumscribe both of legs10a and 1012 as a whole, thereby creating a D.C.-conductive closed pathabout legs 10a and 10b as a whole. This path is the equivalent of ashorted turn circumscribing the two legs, but since any net flux in core10 appears to the shorted turn as a flux in leg 10a in one sense, and asthe same flux in leg 10b, but in a sense opposite said one sense, thatflux which is created by primary windings 13 and 23 does not create anynet voltage in one sense or the other in said turn.

On the other hand, the shorted turns defined by shields 14, 24, and 34have several beneficial effects. One of these is that the shields strapthe transformer, namely, a common transformer practice is tocircumscribe the transformer with a single band of copper, or the like,so as to define a shorted turn about the core of the transformer as awhole, for the purpose of cutting down flux leakage from the transformercore. It will be appreciated that flux leakage can create unbalancedconditions in the transformer resulting in undersired voltages due touneven flux distribution, and/or unintended linkages between coils dueto stray flux.

The. foregoing aspect of the shields 14, 24 and 34 is electromagnetic innature as opposed to their electrostatic properties, which Would berelatively little affected if the closed turns were cut through so as toprevent circulation of a current around the core 10 via the shield.

Another aspect of the shields has to do with the core to winding and/orshield capacitance. If this capacitance couples an interfering currentthrough the transformer via the two legs 10a and 10b of core 10, theshorted turns of the shields will induce a counter-current in said legsthat will reduce the net effect of the interfering currrent. This,again, is an electromagnetic effect of the shields.

It is to be noted that it is often as necessary to so confine themagnetic field of the transformer, as it is to keep external fields outof the transformer, which is frequently housed in Mumetal, or likeexternal magnetic shielding, for this dual purpose. Since the strappingeffect of shields 14, 24 and 34 aids in this insofar as the transformerfield is concerned, the degree of shielding effect required of theexternal magnetic shielding, is substantially reduced.

As FIGURE 1 suggests, the shielding configuration necessary to fulfillthe functions set out above is obtained most conveniently by means ofbobbin-type coils, that is, coils which are each wound on its own form,spool or bobbin, which are then mounted side by side on the core legs.This is to be contrasted to layer-style winding wherein the individualcoils are wound in layers, one on top of the other with interleavingshielding between winding layers, as may be appropriate, and often onbut one leg of an EI core, in an attempt to secure some degree offreedom from interference in the form of intershield current. While itis evident that the windings and shields of FIGURE 1 could be laid oneon top of the other without altering the electrostatic shielding effectof the shields, the path or paths of the capacitative current betweenthe shields would be at right angles, more or less, to that taken bysaid current in the transformer according to FIGURE 1. As a result, anet flux would be created in the core, for the capacitative current nowwould be directed in and out of the plane of FIGURE 1 with the resultthat whether the current path were between the legs a and 10b, and/or toone side or another of either or both legs 10a and 10b, additive fluxeswould be induced in the core 10.

Various proposals have heretofore been made, in the way of layer-typewinding and shield arrangements, in which the intershield current issupposed to be neutralized or compensated for.

However, I have found it is most difficult to obtain effective shieldingand balance of the transformer structure by following these prior artproposals, and practically impossible to do so on a mass productionbasis. In contrast, the principles of my invention, as shown in FIG- URE3, can be implemented by the coil unit shown in FIGURES 2 et al., whichis easy to produce on a large lot basis, yet may be used to producewell-balanced and efficiently-shielded transformers, with a minimum ofskill and effort devoted to adjusting individual transformers foroptimum interference rejection properties.

The basis of the unit shown in FIGURES 2 et al. is a dual coil windingwherein the coils are mounted side by side in a sort of figure-8 shieldhaving a slot, insulated imbrication, or like interruption, completelythrough the node or septum where the upper and lower bight of thefigure-8, cross or merge, respectively. The structural realization ofthis form is best apprehended from consideration of the exploded view ofFIGURE 2, wherein reference numerals 11, 12 and 13, respectively, denotethe half-primary 11, and its coils 12 and 13, of FIGURE 1. Coils 12 and13 are wound on suitable forms such as square bobbins 12a and 13a, ofmolded nylon, or other suitable material. Shield 14 of FIGURE 1 isrepresented by side-plates 14a and 14b, a pair of square ferrules14c and14d, and a side-plate spacing band 14e. Side-plates, side-plate spacingband and ferrules must be made of D.C.-conducting material, preferablyvery thin sheet copper, or like material, the D.C.-resistance of whichis low enough to neglect. Thickness of the metal parts has beenconsiderably exaggerated for ease in rendering it in the figures.

Side-plates 14a and 14b are octiform, that is to say, each is shapedlike a figure-8, and each is slotted through the septum of the figure-8,as shown at 16a and 16b, respectively. Moreover, ferrules 14c and 14dare slotted along their lengths as shown at 16c and 16d, respectively.Finally, the four corners of each of side-plates 14a and 14b arenotched, :as exemeplified at 14g on side-plate 14b, and the ferruleseach have their six corners (being open at both ends, each would haveeight corners, four at each end, save that the slot therein takes in twoof the eight) slit or notched out, as exemplified in heavy black line,both at 14f of ferrule 14d, and at the other of the said six corners ofeach ferrule.

The exploded parts so dimensioned that if they are compressed, to thestate indicated in FIGURE 3, sideplates 14a and 14b, ferrules 14c and14d, and spacing band 142, form a complete envelope for the winding 11,including coils 12 and 13, and their bobbins 12a and 13a, except forslots 16a through 16a, and a hole 14h in spacing band 14a.

As indicated, the coils 12 and 13 are square-wound, hence windows orcut-outs 12b and 13b in the bobbins 12a and 13a, are square, too, as arethe corresponding windows or cut-outs in side-plates 14a and 14b, andthe corresponding passages through ferrules 14c and 14d.

As is evident from FIGURE 3, the notches 14f, 14g et al., of FIGURE 2,create tabs along the peripheries of side-plates 14a and 14b, and alongthe ends of ferrules 14c and 14d, that may be bent over, as exemplifiedat 141, 1412, and 14 whereby the ferrules 14c and 14d clasp spacer band14c and side-plates 14a and 14b, and sideplates, ferrules, and spacerband form one continuous whole insofar as DC. conduction is concerned,by reason of the numerous metal to metal contacts made at theperipheries and windows of the side-plates 14a and 14b, the edges of theband 14e and the ends of ferrules 14c and 14d.

However, the slots 16a through 16d, join end to end, so that the DC.path from the vicinity of holes 14k to 14k, which latter marks the placewhere the strip of metal defining the space band has its ends solderedtogether to close the spacer hand, does not include the septum of theoctiform shape, that septum being, in the assembled shield or shieldedwinding module, the composite of the next adjacent sides of ferrules 14cand 14d, and the septa separating the windows of side-plates 14a and14b. The composite septum, thus defined, is thus slotted completelythrough by the composite slot 16, composed of slots 16a through 16a.

The hole 14h in spacer band 14e provides for the coilleads shielding 15which, as shown, is tubular flexible braid, preferably copper, or thelike, the end filaments of which are splayed over around thecircumference of hole 14k, and are soldered to the spacer band Me.

A pair of open-ended square ferrules 17a and 17b made of insulatingmaterial are provided so that the core legs 10a and 10b of core 10 cansupport the half-primary 11, without being in electrical contacttherewith.

The slot 16, it will be noted, is located at one extremity of the septumof the shield 14, and the leads 18 of the primary-half are locateddirectly opposite, but like the slot, symmetrically disposed withrespect to the windows in the shield and core assembly, which windowsare formed by the square passages through ferrules 17a and 17b. Coils,ferrules, side-plates and spacer band are bilaterally symmetric inshape, number of turns, etc., with respect to a plane A-A extending inthe line of the explosion of parts in FIGURE 2, which plane would alsobe a vertical bisector of core 10, normal to the view adopted in thecase of FIGURE 1.

FIGURE 4 shows the entire transformer that would result in adopting theconstruction shown in FIGURES 2 and 3 for all three windings, 11, 21 and31. The complete assembly also includes spacer plates 19a, 19b, 20a and20b, there being provided like endmost plates 19a and 19b havingsubstantially the form of side-plates 14a and 14b, and inner spacingplates 20a and 20b of like form except that, as shown in FIGURE 5, holessuch as indicated at 20c, 20d and 20s, may be provided for purposes tobe indicated hereinafter. Core 10 may be of the usual shelltype whereinthe core is formed by stacking windowed, rectangular laminations havingone side missing, and butting these with a stack of solid laminationscorresponding to the missing sides to substantially confine the magneticcircuit in a ferrous loop.

The four plates 19a, 19b, 20a and 2% are made of insulating material sothat, as will be evident from FIG- URES 3 and 4, the shielding 14, 24,and 34 is insulated from each other by plates 20a and 20b, and from coreby plates 19a and 19b and ferrules 17a and 17b.

Also visible in FIGURE 4 are the bent-over edges 24i, 24n, 341 and 34ncorresponding to like bent-over edges 14i and Mn of side plates 14a and14b, the side plates to which said edges 241' et al. belong not beingvisible otherwise in FIGURE 4.

Insofar as the shielding 14, 24 and 34 is concerned, each is identicalto the other, hence, as FIGURE 4 indicates, the shielding 34 isreversely oriented on the core 10 with respect to shielding 14 and 24.Each of the shielding 14, 24 and 34 is bilaterally symmetric withrespect to a plane B B extending in the line of the exposed parts inFIGURE 2, which plane would be the horizontal bisector of core 10 normalto the view adopted in FIGURE 1, except for hole 14h and itscounterparts (not shown) in shielding 24 and 34, joint 14k and itscounterparts (not shown) in shielding 24 and 34, and slot 16 and itscounterparts (not shown) in shielding 24 and 34, and winding leadshielding 15, 25 and 35. Hence, the nonsymmetric elements of shielding34 (and of the coil leads of the winding therein), therefore are locatedon the one side of plane BB, and the corresponding non-symmetricelements of shielding 14 and 24 are located on the other side of theplane B-B and, except for this, bilateral symmetry obtains with respectto plane BB.

The foregoing construction has quite substantial consequences insofar asis concerned ease of manufacture and assembly, and of predeterminationand adjustment of electrical symmetry.

The creation of the shield and winding assembly of FIG- URES 2 and 3would begin with a pair of identical bobbins 12a and 130, which we maysuppose supported in the position shown, but somewhat spaced to permitthe coils to be wound therein. Next, envisage winding each bobbin in asense counter to each other and terminating the winding starts andfinish next adjacent each other, so that if we connect the start leadsof each coil together and connect the series-connected coils by theirfinish leads across a DC. source, the magnetic field of one coil wouldbe exactly opposite the other, and both fields would be parallel toplane AA.

The coils, wound in this way, then, are supported together as shown inFIGURE 2 with the next adjacent fiat edges of their bobbins flush upagainst each other. The start leads are then soldered together andpigtail leads soldered to the finish leads. The exposed layers of wireare than overlaid with insulating tape (following the path indicated byband He, but between the bobbin flanges) passing on both sides of thepigtail leads so that these are centered with respect to the two coils,and depart straight-away therefrom. The pigtail leads are twisted fornoise reduction and are coded to permit identifying the individualfinish leads. If desired, a perforated strip of mica (not shown) may belaid between the flanges on one side of the taped coil-pair, thepigtails passing through the perforation in the strip, in order to helpcenter the pigtails and protect the coil insulation and insulating tapein the vicinity of the soldered joints in this area from the heat ofsoldering operations on the shield structure to be applied.

Spacing band 14:2, in the form of a flat strip of tinned .005 copper,say, and having hole 1411 and braided shielding affixed thereto, asillustrated in FIGURE 2, is bent around the taped coils and soldered atthe overlap denoted by reference numeral 14k, the pigtails beingreceived in said hole and braided shield 15. Preferably the strip iswide enough to rest on the edges of the bobbin flanges. Then, theferrules 17a and 17b may be fitted into the windows 12b and 13b of thetaped and banded coil assembly. The ferrules will be cut long enoughthat their ends project from both sides of the said windows, about thedepth of the notches 14f, whereby the projecting parts of the ferrulesdefine the tabs noted before.

Side-plates 14a and 14b are then fitted to the taped, banded andferruled coil assembly with the aforesaid tabs projecting from theside-plate windows and more or less flush against the edges thereof. Thetabs are then bent over against the side-plates, pressing the latterfirmly against the flanges of the coils, and a few, or all thereof, aresoldered to the side-plates to assure D.C. continuity between ferrulesand side-plates. The side-plates are so dimensioned as to overlap theouter contour of spacer band 14:: by about the distance of notches 14get al., whereby to define tabs, that are then bent over against theouter surface of the spacer band, and soldered to the spacer band atseveral places or substantially all along the seams therebetween. Ifplace-wise soldering is used, in either or both of the ferrules andside-plates, and the side-plates and ferrules, the symmetry ofconstruction followed thus far should be observed, though it is notstrictly essential if the materials used are copper or likehighlyconductive material and highly-conductive solder joints can bemade. Of course, if good D.C. contact can be made without soldering, sayby mere pressure of the assembled transformer parts on each other, bymeans of conducting paint, adhesive, or the like, soldering may beentirely dispensed with. However, wherever contact is poor or absent,there is a possibility of high resistance or electrostatic leakage,which may electrically unbalance the assembly.

If the transformer to be constructed were to be 1:1 in turns ratio(secondary turns versus half-primary turns), the several windings 11, 21and 31 may each be exactly identical to the others. Moreover, for ratiosother than unity, lows, though not like the secondaries. This is becausethe mode of winding allows the electromagnetic orientation of thehalf-primaries on the core to be taken care of by interconnecting thehalf-primaries and chopper, in the manner shown in FIGURE 6, whereincertain of the elements of FIGURE 1 are reproduced in some casesfragmentarily, and the reference device R and transducer T have beenreplaced by a DC. source E. In this figure, the reference characters Findicate the finish leads of the several coils 12, 13, 22 and 23, theselatter being variously subscribed to the characters F in order todistinguish among the different finish leads. Likewise, the characters Gare similarly subscribed to permit distinguishing the several startleads.

It will be recalled that the pigtails of the coils were coded to permitidentification of coil leads. We see then that finish lead F of coil 12is in effect a pigtail connected to terminal 4, the center tap of thetransformer primary and the negative terminal of source E. Continuing inthis vein, start G of coil 12 and G of coil 13, is the solderedconnection made to begin with in producing the coil assembly. Finishlead F then, is in effect, a pigtail connected to contact 1 of modulatorS.

Again, the start leads G and G of coils 22 and 23 would be the solderedcoil connection made in producing this coil assembly, and finish leads Fand F would be the respective pigtails applied to the finish leads ofcoils 22 and 23. In this case, however, the pigtail of finish lead F ofcoil 22, is connetced to contact 2 of switch S and the pigtail of thefinish lead F of coil 23 is connected to the center tap 4, that is,electromagnetically speaking, the finish leads F and F correspond, inthat order, to finish leads F and F in that order.

As a result, if source E is imagined to drive a current from itsposition terminal, through chopper S and the coils, to its negativeterminal and center tap 4, upon following the arrow heads showing thealternate current paths via contacts 1 and 2, it is seen that the fluxin legs each half-primary may still be exactly like its fela and 10b isin one sense or the other depending on which of contacts 1 and 2 is madeby contact 3. Thus, making contact 2 creates a flux having the sense ofthe arrow at the lower end leg 10a which continues as indicated inbroken line, on to leg 1%, where its sense is in that of the arrow onthe fragment of 10b shown inside coil 23. When contact 1 is made,however, the arrow on the top portion of leg 10a, the broken linecontinuation thereof to the arrow on the portion of leg 10b inside coil13, and the last said arrow, indicate that the flux in core 10 isreversed in its clock sense from the clock sense it had when contact 2is made. This is the desired action of course.

The slot 16, and its counterparts (not shown) on the shielded Windingmodules 24 and 34, is a window through which the several coils see eachother, and the core, etc., so to speak, via distributed capacitancedirectly between coils, coils and core, etc. The end to be attained bythe shielding, however, insofar as possible, is to assure that no partof the transformer but the shielding directly involves the coils in adistributed capacitance. It is therefore desirable to reduce theresidual capacitance directly involving the several coils by cappingwindow 16, and its said counterparts 'With a slot cover 30, such as isshown in FIGURES 7 and 8 prior to assembling the winding modules on thetransformer core (a portion 30a of said cover is visible on modules 14and 24, as indicated in FIGURE 4).

As FIGURE 7 indicates, cover 30 is simply a more or less U-sha-ped strip30a having arms 30b at one end there-of, the said arms and both ends ofthe strip covering and extending beyond the several parts of slot 16,tabs 30c being provided that are bent over the end of strip 30a, flanges30d being provided to assure overlap of the slots 16d of ferrules 14cand 14d, and the cover 30, which would normally be of the same D.C.conductive material as spacer band 14c, being solder-tacked to thelatter at 308'. It is necessary that the cover 30 be insulated from theportion of the shieldseptum extending upward toward slot 16, hence, afew wraps of insulating tape 30 are wound around the last said portionto prevent D.C. contact between cover 30 and said portion of saidseptum. The tap wraps are shown partly in dotted line. The coil 12 andbobbin 12a and part of the corresponding shielding have been omitted forclarity in FIGURE 8.

'Other shielding material that would be suitable are foil, metallized orfoil-coated paper. For instance, paper-backed foil is quite convenient,since it can be fitted snugly to the contours of the bobbin-pair 12a and13a, using suitable adhesive to adhere together pieces, quite similar tothose from which the illustrated shielding elements are formed into ahomogenous envelope, wherein D.C. conductivity between seams is assuredby overlapping the said seams' with a layer of DC. conductive paint. Inthis type of construction, the foil side of the shielding material wouldbe on the outside. The sheet metal shielding envisaged, supra, is stiffenough to require forming tools to shape, and is not readily maintainedin its final shape on the bobbins without soldering. The more flexiblematerial requires no forming tools, holds the shape given it under meresmoothing with the fingers, andeliminates soldering and itspossibilities for thermal damage to winding insulation, etc.

The bobbin pair could also be shielding by dipping in DC. conductivepaint, after a preliminary dipping in some insulating resin, paint, orthe like, the slot 16 being formed afterward by stripping the pain inthe corresponding area, covering the slot with insulating tape, as inthe case of the species of FIGURE 8, and painting over the tape from apoint spaced from one edge of the tape wrapping to and over onto theoriginal conducting layer bounding the other edge of said tape wrap.

The construction illustrated in FIGURES 2 to 5, inclusive, is inherentlysymmetric, and this symmetry,

which is a crucial factor in obtaining the highest degree ofinterference rejection, is readily obtained to almost any desired degreeof perfection in a routine manner. Furthermore, though the tolerancesinvolved in stamping out parts, in assembling the winding units andtransformers, and so on, may be such that some undesirable degree ofassymmetry is displayed by a finished transformer, which symmetryadversely affects the interference-rejecting ability of the finishedtransformer, such assymmetry is easily compensated for on a lot basis.This is done by adjusting the size of the holes in the spacers 20a and20b, as for example, holes 20c, 20d and 20e in spacer 20a. These spacersform part of the dielectric of the distributed capacitance betweenshields 14 and 24, 24 and 34. Adjusting hole sizes relative to eachother is, in effect, treating the intershield capacitance as if itconsists of three paralleled branches of capacitors, one such branchbeing centrally through the transformer between and parallel to corelegs 10a and 10b, another branch being virtually along leg 10a and thethird being virtually along leg 10!). While such adjustment actuallyonly affects the distributed capacity in the immediate vicinity of theholes in the spacer, it provides, to a sufiicient approximation, theeffect of redistributing the intershield capacitance to the extentdesired, and, for any given production lot of transformers, theadjustment of shield-hole size or sizes in one transformer thereof, onethereof in general suits the needs of the others of the lot, i.e., theadjusted spacers are the same in every transformer of the lot. Where thespacers 20a and 2% are initially provided with three holes asillustrated, obviously, one, several or all of the holes of one or ofboth spacers may require adjustment in a given case.

The spacers 20a and Zfib also may be initially provided without suchholes, which then are only supplied, in such manner and in such size asis indicated by testing a sample transformer of a given lot to see whatthe lot needs in the way of adjustment of intershield capacitance. Forthe purposes of such test, of course, all four spacers 19a, 19b, 20a and20b of the sample transformer initially would be imperforate (save forthe core-leg receiving windows therein, of course).

In any event, the test involved in both cases is basically to determineto what extent an A.C. voltage, across the capacitance between shields14 and 24 in parallel with the capacitance between shields 14 and 34, isreflected in the voltage across winding 31. For example, one terminal ofan A.C. source may be connected to both shielding 15 and 25, and theother terminal thereof to shielding 35. Any difference between thevoltage across winding 31 when the A.C. source is thus connected, andthe voltage across winding 31 with the A.C. source disconnected, will belangely due to assymmetry of the distributed capacitance between shields14, 24 and 34.

In the claims appended hereto, I have freely used the terms octiform andfigure-8, without intending any distinction between the two terms, otherthan that the former is a convenient way to express the latter conceptas an adjective and less strongly suggests the typographical aspect ofthe term figure-8.

It is to be understood that the term figure-8 is used to suggest theessence of the configuration thus expressed rather than the minutiae oftypography and script. For my purpose, a figure-8 shaped, or octiformentity is any discrete body or discrete portion of a body, wherein saidbody or portion of a body has any arbitrary contour, surface, envelope,etc., and said contour, surface, envelope, etc., is pierced by a pair ofholes each of which is distinct from the other and do not communicatewith one another save through the region external to said contour,surface, envelope, etc., unless a slot or the like is provided betweenthe holes after the fashion indicated in the case of slot 16b ofsideplate 14b, slot 16d of ferrule 14d, and the composite slot 16 (asdefined in connection with the description, supra, of FIGURE 3).

The portions of the material surrounding or defining such holes in eachcase form a bight, and the ends of each bight merge with each other at acommon node, i.e., in a septum, so that the figure-8 or octiformconfiguration is composed of a pair of bights and a septum joining theends of the bights. While the aforesaid slot interrupts the septum andprevents it from literally merging the ends of the bights, theinterruption of the equi-potential surface, as such, is small, and ifthe treatment of FIG- URES 7 and 8 be followed, practically nil, exceptas regards prevention of shorted turns about legs 10a and 10bindividually. Hence, despite the slotting, the bights may be said to bemerged by the septum. Thus, within the range of the terms octiform andfigure-8 are the configurations illustrated in FIGURES 9, l and 11showing views of respective octiform configurations 50, 51 and 52 whichmay be taken to represent alternate configurations of the bobbin pair12a and 1301, or of the coils thereon, the reference characters 50a,50b, 51a, 51band 52a and 52b, representing the several windows or holesthrough the bobbins. Note that in FIGURE 11 the corresponding shieldingwould have a septum of very small extent and that slotting such septumwould more or less extirpate it, depending on how closely the shielding(indicated at 54 in dashed line; as is the slot thereof, denoted byreference numeral 56) follows the circular contours of the bobbins ofthe pair 52.

Having fully described my invention both in principle and in the bestform thereof known to me thus far, and, as well the manner of making andusing my invention, I claim:

1. A transformer having a closed core consisting essentially offerromagnetic material providing a substantially continuous closed lowreluctance path and including first and second ferrous legs, said legsextending parallel to and spaced from each other; a primary windingincluding first and second coils, a secondary Winding including firstand second coils; each of said first coils having its turnscircumscribing one of said legs and said first coils being supportedside by side on said one of said legs; each of said second coils havingits turns circumscribing the other of said legs and said second coilsbeing supported side by side on said other of said legs, first D.C.conductive shielding having an octiform surface separating said firstand second coils of said primary winding from said secondary winding;second D.C. conductive shielding having an octiform surface separatingsaid first and second coils of said secondary Winding from the saidoctiform surface of said first D.C. conductive shielding; the saidshieldings being insulated each from the other, and each thereofsurrounding each of said legs individually and both said legs as awhole, so that each said shielding has a figure-8 configuration thebights of which circumscribe said legs and merge in a septum lyingbetween said legs, said bights and said septum defining said octiformsurface of each said shielding.

2. The transformer of claim 1, wherein the said septum has aninterruption in the nature of a slot extending from and through the sideof said septum next adjacent one of said legs to and through the side ofsaid septum next adjacent the other of said legs; the location of saidinterruption being such as to preserve a D.C. conductive path about thesaid core as a whole, via said bights, while eliminating D.C. conductivepaths about the individual legs of said core, via said septum.

3. The transformer of claim 1, wherein the one said shieldingsubstantially completely envelops the corresponding one of the saidwindings; and the other said shielding substantially completely envelopsthe other of the said windings.

4. The transformer of claim 1, wherein each bight of one said shielding,and the septum thereof, is tu'bular, and such bights open into oneanother and into such septum; said first coil of one of said windingsbeing enveloped in part by the septum 0f the one said shielding, andsaid sec 0nd coil of said one of said windings being enveloped in partby the last said septum, the remainder of the last said first coil beingenveloped by one bight of the said one said shielding and the remainderof the last said second coil being enveloped by the other bight of thesaid one said shielding; the other said shielding being like the saidone said shielding and enveloping the other of said windings like thesaid one said shielding envelops the said one of said windings.

5. Transformer structure for use with a chopper and having a primarywinding wherein each half thereof is, cyclically, switched into and outof a low-impedance circuit, and out of and into a high-impedancecircuit, the arrangement being approximately that neither one of suchhalves, nor the pair thereof is, in both said circuits simultaneouslyand/orout of both said circuits simultaneously, said transformerstructure including, in combination, a closed core consistingessentially of ferromagnetic material providing a substantiallycontinuous closed low reluctance path and said core having a pair of legportions, a primary winding wound on said core, and a secondary windingon said core; said primary winding including a first pair of coils, onecoil thereof being on one of said leg portions and the other coilthereof being on the other of said leg portions; said primary windingalso including a second pair of coils, one coil thereof being on one ofsaid leg portions and the other coil thereof being on said other of saidleg portions; said secondary Winding including a third pair of coils,one coil thereof being on one of said leg portions and the other coilthereof being on the other of said leg portions; each said leg havingthe coils thereon mounted thereon side by side; the coils of said firstpair being connected to each other and wound in a sense such as toconstitute a said half of a primary winding, and to be in seriesaidingrelation relative to voltages induced therein 'by a magnetic flux in aclosed flux path including both said leg portions in series; the coilsof said second pair being connected to each other and wound in a sensesuch as to constitute another said half of a primary winding, and to bein series-aiding relation relative to voltages induced [herein by saidmagnetic flux; said one said half and said another said half beingconnected together in seriesopposing relation relative to voltagesinduced therein by said magnetic flux; said secondary winding beingbetween the said halves of said primary winding; first D.C. conductiveshielding having an octiform surface separating said first pair of coilsof said primary winding from said coils of said secondary winding; saidfirst D.C. conductive shielding having another octiform surfaceseparating said second pair of coils of said primary winding from saidcoils of said secondary winding; second D.C. conductive shielding havingan octiform surface separating said one octiform surface from said coilsof said secondary winding; said second D.C. conductive shielding havinga further octiform surface separating said another octiform surface fromsaid coils of said secondary winding; said shieldings being insulatedeach from the other, and each thereof surrounding each of said legsindividually and both said legs as a whole, so that each said shieldinghas a figure 8 configuration, the bights of which circumscribe said legsand merge in a septum lying between said legs, and a pair of said bightsand a said septum defining each said octiform surface.

6. A transformer having a closed core consisting essentially offerromagnetic material providing a substantially continuous closed lowreluctance path and including first and second ferrous legs, said legsextending parallel to and spaced from each other; .a primary windingconsisting of first and second primary winding halves each includingfirst and second coils; a secondary winding including first and secondcoils; each of said first coils having its turns circumscribing one ofsaid legs and said first coils being supported side by side on said oneof said legs; each of saidsecond coils having its turns cirscribing theother of said legs and said second coils being supported side by side onsaid other of said legs; first D.C. conductive shielding having oneoctiform surface separating said first and second coils of said firstprimary winding half from the said coils of said secondary winding; saidfirst D.C. conductive shielding having another octiform surfaceseparating said first and second coils of said sec-nd primary windinghalf from the said coils of said secondary winding; second D.C.conductive shielding having one octiform surface separating said firstand second coils of said secondary Winding from said one octiformsurface of said first D.C. conductive shielding; said second D.C.conductive shielding having another octiform surface separating saidfirst and second coils of said secondary winding from said anotheroctiform surface of said first D.C. conductive shielding; said firstcoil of said secondary winding being positioned between said first coilsof said primary winding halves and said second coil of said secondarywinding being positioned between said second coils of said primarywinding halves; the said shieldings being insulated each from the other,and each thereof surrounding each of said legs individually and bothsaid legs as a whole, so that each said shielding has a figure-8configuration the bights of which circumscribe said legs and merge in aseptum lying between said legs, and a pair of said bights and a saidseptum defining each said octiform surface.

7. The transformer of claim 6, wherein the said septum has aninterruption in the nature of a slot extending from and through the sideof said septum next adjacent one of said legs to and through the side ofsaid septum next adjacent the other of said legs; the location of saidinterruption being such as to preserve a DC. conductive path about thesaid core as a whole via said bights, while eliminating D.C. conductivepaths about the individual legs of said core, via said septum.

8. The transformer of claim 6 wherein one said winding is substantiallycompletely enveloped by the corresponding said shielding; and the othersaid winding is substantially completely enveloped by the other saidshielding.

9. The transformer of claim 6, wherein each said octiform surface of onesaid shielding is defined by tubular means of figure-8 form having apair of bights and a septum, wherein each of said bights is tubular,said septum is tubular and said bights merge with and open into saidseptum and each other; the said first and second coils of thecorresponding said winding being substantially enveloped by said tubularmeans, the arrangement being that a said first coil is enveloped in partby the last said septum, and a said second coil is enveloped in part bysaid last said septum; the remainder of the last said first coil beingenveloped by one said bight opening into and merging with said last saidseptum and another said bight, and the remainder of the last said secondcoil being enveloped by said another said bight opening into and mergingwith said last said septum and the said one said bight; the other saidshielding being defined by tubular means of a form like the tubularmeans of the said one said shielding; the said tubular means of saidother said shielding enveloping the said first and second coils of theother said winding like the tubular means of the said one said shieldingenvelops the said first and second coils of the corresponding saidwinding.

10. The transformer of claim 6, including octiform spacers of insulatingmaterial, each said spacer surrounding each of said legs individuallyand both of said legs as a Whole, so as to have a figure-8configuration, the bights of which circumscribe said legs and merge in aseptum lying between said legs, said spacers being positioned betweensaid shielding, and separating the said surfaces of one said shieldingfrom the said surfaces of the other said shielding; each said spacerhaving the amount of material in one said bight thereof opposite saidseptum, the amount of material in the other said bight thereof oppositesaid septum and the amount of material in said septum so proportionedwith respect to each other as to compensate for lack of uniformity indis tributed capacitance between said shieldings.

References Cited by the Examiner UNITED STATES PATENTS 873,036 12/1907Frank 336-184 X 1,117,293 11/1914 Wilson 336-216 X 1,624,560 4/1927Payne 336--84 X 2,911,604 11/1959 Krause 336-92 X 2,945,216 7/1960 Gygeret al 33684 X LEWIS H. MYERS, Primary Examiner.

JOHN F. BURNS, ROBERT K. SCHAEFER,

Examiners. W. M. ASBURY, Assistant Examiner.

1. A TRANSFORMER HAVING A CLOSED CORE CONSISTING ESSENTIALLY OFFERROMAGNETIC MATERIAL PROVIDING A SUBSTANTIALLY CONTINUOUS CLOSED LOWRELUCTANCE PATH AND INCLUDING FIRST AND SECOND FERROUS LEGS, SAID LEGSEXTENDING PARALLEL TO AND SPACED FROM EACH OTHER; A PRIMARY WINDINGINCLUDING FIRST AND SECOND COILS, A SECONDARY WINDING INCLUDING FIRSTAND SECOND COILS; EACH OF SAID FIRST COILS HAVING ITS TURNSCIRCUMSCRIBING ONE OF SAID LEGS AND SAID FIRST COILS BEING SUPPORTEDSIDE BY SIDE ON SAID ONE OF SAID LEGS; EACH OF SAID SECOND COILSHAVINGITS TURNS CIRCUMSCRIBING THE OTHER OF SAID LEGS AND SAID SECONDCOILS BEING SUPPORTED SIDE BY SIDE ON SAID OTHER OF SAID LEGS, FIRSTD.C. CONDUCTIVE SHIELDING HAVING AN OCTIFORM SURFACE SEPARATING SAIDFIRST AND SECOND COILS OF SAID PRIMARY WINDING FROM SAID SECONDARYWINDING; SECOND D.C. CONDUCTIVE SHIELDING HAVING AN OCTIFORM SURFACESEPARATING SAID FIRST AND SECOND COILS OF SAID SECONDARY WINDING FROMTHE SAID OCTIFORM SURFACE OF SAID FIRST D.C. CONDUCTIVE SHIELDING; THESAID SHIELDINGS BEING INSULATED EACH FROM THE OTHER, AND EACH THEREOFSURROUNDING EACH OF SAID LEGS INDIVIDUALLY AND BOTH SAID LEGS AS AWHOLE, SO THAT EACH SAID SHIELDING HAS A FIGURE-8 CONFIGURATION THEBIGHTS OF WHICH CIRCUMSCRIBE SAID LEGS AND MERGE IN A SEPTUM LYINGBETWEEN SAID LEGS, SAID BIGHTS AND SAID SEPTUM DEFINING SAID OCTIFORMSURFACE OF EACH SAID SHIELDING.